Serial bus systems sometimes have a limited address space so that the number of addresses available has to be carefully managed. An example of this is Profibus according to the standards IEC 61158 and IEC 61784. Profibus has a maximum of 253 addresses available, some of which are reserved, and node devices can be operated using the remaining addresses. Profibus is widely used in industry for automation systems with decentralized peripherals.
Node devices for a Profibus system have a Profibus node, a PCB, also known as a Backplane, and sometimes also connections for input/output cards. The node device may take up a maximum of 16 addresses, due to a standardization or restriction resulting from the Profibus standard.
The address space of 16 addresses that is available for each node device is generally adequate if conventional modules are connected via the input-output modules, since in this case the address space required for the individual input/output card is limited. For this reason, a plurality of input/output cards can generally be connected to a node device, said cards dividing the available address space between them.
If a complex functional module is to be controlled within an automation system, then this module is connected to the node device via an input/output card and requires for this purpose an address space which is sometimes equivalent in size to the address space for the node device as a whole. Depending on the complexity of the functional module, there is therefore possibly no longer the option of connection to the node device, in particular if the input/output card on which the complex functional module is connected has to share addresses with other input/output cards.
It clearly emerges from the foregoing that connection problems arise if only a limited address space is available to a node device. This limitation of the address space can arise here because of a standard, as in the case of Profibus.
To solve this problem, it has been suggested that each node device could be provided with an address and the input/output values of the input/output modules could be shown on the register of the node device. This requires complex software in order to produce a registering map. If, for example, a target speed for an electric motor connected to an input/output card is to be determined, this speed is then recorded in the register for the node device and is then transmitted from the register to a dual-port RAM of the input-output module. From there the target value is transmitted to the electric motor. The reverse applies accordingly to reading off the target values for the speed of said electric motor. This architecture is found, for example, in multi-motor drives, in which the individual drive devices are connected in a string or in several strands via Profibus.
A further known approach to solving the aforementioned problem consists in equipping each node device with a separate address- and data bus that is different from the Profibus. The input/output cards thereof are then connected to said address- and data bus of the node device. As a result, the address space in the individual node devices is vastly increased, the address space being quickly assigned to the slots. This means that one or a plurality of addresses is/are always reserved for a slot to which an input/output card is connected, even if no input/output card is actually connected.
U.S. Pat. No. 5,978,578 describes a field bus system to which input/output cards are connected. Input-output devices such as sensors, motors, monitors, machines and so on are connected to an input/output card that has a control bus available. The control bus is designed such that devices with an ISA, PCI, or Compact PCI bus or other bus can be connected thereto.